Horticultural luminaires are widely used in greenhouse installations for growing flowers, tomatoes and other horticultural products. Usually the luminaires are suspended by a pair of wires from an overhead support such as a rafter, or other bearer, so that the luminaire is substantially horizontal. As a consequence, the light produced by the luminaire shines downwardly on the plants below.
The lamps used in such horticultural applications are high intensity discharge (HID) lamps such as high pressure sodium lamps which have a negative resistance characteristic. As a consequence, a ballast is required to operate the lamp and control the lamp current. Traditionally such ballasts have been inductors and the lamps have been operated at AC mains frequency. As a result, the ballast can be mounted in any convenient position (often approximately at waist height) and is connected to the reflector and lamp by means of a cable having a length in the vicinity of 2-3 m (6-9 ft).
However, in recent times, electronic ballasts have become increasingly popular. Such ballasts operate at high frequencies and are therefore liable to cause radio frequency interference (RFI). In order to reduce the possibility of RFI it is desirable to shorten the electrical connection between the ballast and the lamp. As a consequence, it is desirable to mount the ballast immediately adjacent the lamp. Luminaires where the ballast is immediately adjacent the lamp are said to be close coupled.